Overdoing It on Fish Oil May Raise Men’s Cancer Risk

A new study finds that high levels of omega-3 can increase prostate cancer rates

Men and women around the world have come to see omega-3 fatty acids, most commonly found in fish oil, as essential to their diets.

Years of research have shown that the acids are important for brain function and play a key role in reducing one's risk of heart disease but that too many people don't get enough of them through diet alone.

However, a disturbing new study released this week shows that men who seek to boost their omega-3 levels by eating more fish or taking fish-oil supplements may actually increase their risk of contracting the most aggressive form of prostate cancer — and they may not even be getting the promised heart benefits.

The study, published by the Journal of the National Cancer Institute, relied on data from a large, long-term prostate cancer trial called SELECT (Selenium and Vitamin E Cancer Prevention Trial). Its research team analyzed the health and diet of 834 men in the study who had developed prostate cancer and 1,393 others, chosen at random, who had not. (The study involves more than 35,000 men overall.) The researchers then divided the men into groups based on their blood levels of EPA, DPA and DHA — the anti-inflammatory fatty acids found in fish and fish-oil supplements — and found that those with the highest levels had a 71 percent higher risk of high-grade prostate cancer, the type most likely to involve fatal tumors, than those with the lowest levels. Men with the highest levels also appeared to experience a 44 percent increase in their risk of low-grade prostate cancer.

The difference in omega-3 levels between the men with the highest and lowest counts was slightly more than the equivalent of two meals of salmon per week, according to the authors. Omega-3 fatty acids from vegetable sources such as flaxseeds, olive oil, avocados and walnuts did not have any affect on prostate cancer risk in the study.

The new findings confirm concerns raised by the lead authors, from the Fred Hutchinson Cancer Research Center in Seattle, in a similar 2011 study. "The consistency of these findings suggests that these fatty acids are involved in prostate tumorigenesis, and recommendations to increase long-chain omega-3 fatty acid intake, in particular through supplementation, should consider its potential risks," the authors wrote in the new paper.

Prostate cancer will be diagnosed in about 240,000 men in the United States this year, according to the American Cancer Society. About 30,000 men will die from the disease. The new study's authors emphasized that their research did not analyze the effects of fish oil on men who already have prostate cancer, and acknowledged that separate studies have indicated that omega-3 acids could provide benefits to those men.

The research team was unable to pinpoint exactly why omega-3 fatty acids appear to increase prostate cancer risk. "That's the million-dollar question," says co-author Theodore Brasky, an assistant professor at the Ohio State University Comprehensive Cancer Center.

Mercury, which is also found in fatty fish, does not cause prostate cancer. Omega-3 fatty acids may convert into compounds in the body that damage cells and DNA, but the study's authors agree that more research is needed, especially given that the results are at once definitive and surprising. The widespread assumption had been that omega-3 fatty acids' anti-inflammatory properties delivered a range of health benefits without major risks, other than significantly high doses potentially causing gastric distress or interfering with certain medications. To cite just one example, a recent large-scale research project, known as the MIDAS study, found that older people who got 900 milligrams of DHA daily showed improved memory function after six months compared with those who took a placebo.

Are Supplements Worth It at All?

The new findings, combined with recent research calling into question omega-3 fatty acids' supposed heart benefits, are sure to leave many people questioning their diet choices. A study published in May by the Journal of the American Medical Association raised doubts about omega-3's efficacy in limiting heart attacks. That analysis, of 20 separate studies of people consuming omega-3, and particularly fish-oil supplements, found no significant reduction in rates of heart attack, stroke or premature death for subjects with risk factors for heart disease. It also found no additional benefit from fish-oil capsules for people who were already taking medication to address their risk factors.

"Some of the enthusiasm for these fats has been premature," Brasky told MSN.

The results of the new study will leave many men — and women — searching for answers as they try to craft as healthy a diet as possible. Even with the new data, few if any experts would question the value of fish or omega-3 fatty acids in a balanced diet. But whether adults should take extra measures to boost their intake may be very much in doubt.

"These fatty acids have been promoted as a blanket anti-chronic disease fatty acid," Brasky said. "But nutrition is more nuanced, as is disease occurrence. It's about time we stop talking about foods as good or bad and no gray area."

The study certainly suggests that too many of us may be taking fish-oil supplements we don't need. "We've shown once again that use of nutritional supplements may be harmful," said lead author Alan Kristal, of the Hutchinson Center.

The American Heart Association recommends fish-oil capsules only for people who have high levels of triglycerides in their blood, and other research — which is not called into question by the Hutchinson Center study — suggests that omega-3 supplements may lower the risks of further cardiovascular events in people who have already suffered a heart attack or heart failure.

For the rest of us, the recommended daily value of omega-3 acids is 220 milligrams, easily obtained through two weekly servings of a fatty fish like salmon, mackerel or sardines. "These fish-oil supplements in which some men get mega, mega doses," Brasky told NBC News, "in our opinion is probably a little bit dangerous."